This application claims priority to Korean Patent Application No. 2003-24341, filed on Apr. 17, 2003, in the Korean Intellectual Property Office.
1. Field of the Invention
The present invention relates to digital television (hereinafter, referred to as “DTV”), and more particularly, to an automatic gain controller (AGC) of a digital television (DTV) receiving unit.
2. Discussion of the Related Art
Digital television broadcasts produce higher definition or higher resolution television (HDTV) as compared to traditional analog TV broadcasts. The pictures reproduced from a HDTV broadcast are higher resolution and are less grainy. The images reproduced appear more livelier. DTV signal broadcast formats are defined by the Advanced Television Systems Committee (ATSC). Broadcast signals include vestigial sideband (“VSB”), e.g. 8-VSB for the United States. It is known that broadcasts of VSB signals are susceptible to multipath interference, such as interference from multiple signals arriving at an antenna at different times due to obstacles such as buildings and vehicles. For this reason, there is great interest in the development of a robust receiving unit, capable of stably receiving VSB signals under high-noise multipath channels.
A VSB data frame includes two fields. One field consists of 313 segments. A first segment of each field is a field synchronous signal. The synchronous signal includes 832 symbols. The upper four symbols of the synchronous signal is a segment synchronous signal having levels of +5, −5, −5, and +5. The field synchronous signal indicates a start point of a data field (data+FEC (forward error correction)). The field synchronous signal is used as a reference signal of a channel equalizer, and also is used as a reference signal for determining the use of a NTSC rejection filter (NRF). In addition, the field synchronous signal may be used for confirming channel characteristics, and also may be used for determining loop parameters of a phase tracker.
A field synchronous signal has sequences such as Pseudo-random Number 511 (PN511) sequence, PN63 sequence, VSB MODE, and Reserved. The PN511 sequence is a sequence of a 511-symbol length, which is used as a training sequence for a channel equalizer. The PN63 sequence is a training sequence for a channel equalizer, like the PN511 sequence. In the PN63 sequence, a symbol polarity of a middle PN63 sequence among three PN63 sequences is changed reversely per each field, in order to distinguish a first field and a second field in one frame. The VSB MODE is a sequence of a 24-symbol length, and represents a transmission mode of currently transmitted data. That is, the VSB MODE represents whether the data transmission format is a 15VSB format or a 8VSB format. The Reserved is a space of a 104-symbol length remained as a reserved space.
A conventional digital TV receiver includes an antenna for receiving through the air RF signals, RF and IF amplifiers for amplifying RF and IF signals, and a demodulator for demodulating the IF signals to recover signals representing the broadcasted images. An automatic gain controller (AGC) is used to control the gains of the RF and IF amplifiers. Conventionally, a portion or a digitized version of the IF signal is used as a feedback signal for processing by the AGC. Typically, the AGC controls both RF and IF amplifiers using a feedback loop.
Conventional single feedback loop AGCs which control RF and IF amplifiers with a single integrator have generally been implemented for both coherent and non-coherent modes of operation. The drawbacks to such scheme are twofold. First, the control over amplifiers that have different transfer function characteristics, with a common controlling arm, will result in degraded tracking response, adding to the already non-linear response inherent in practical AGC systems. Second, coherent mode depends on the demodulation processes. Demodulation blocks whose operations are limited and not stable will compromise the performance of the AGC. Other schemes involve the use of signals at the matched filter upward or the equalizer output for decision directed feedback buffer from multi-path interference discussed above, since the coherency of the reference signals cannot always be considered good references for gain control. In addition, the delayed demodulator response prohibits the use of coherent mode of operation with the already time-delayed tuner response that is non-linear.
The use of dual AGCs for RF and IF amplifier control has been proposed. Such device facilitates faster and better gain control as compared to conventional single AGC devices. However, if the RF and IF amplifiers are commonly controlled, even with dual AGCs, the non-linear error characteristics of the amplifiers may be difficult to correct, especially if fixed reference levels are used. For muti-channel processing, different fixed references must also be used.
A need therefore exists for a digital television receiving unit which is adapted to high-noise multiple path channels so as to stably acquire and track VSB signals.